Immunology/Organs of the Immune System

The organs of the immune system are positioned throughout the body. They are called lymphoid organs because they are home to lymphocytes, small white bloodcells that are the key players in the immune system.

There are two groups of immune system organs.

Primary (central)--organs where immature lymphocytes develop
- Thymus
- Bone marrow
Secondary (peripheral)--tissues where antigen is localized so that it can be effectively exposed to mature lymphocytes
- Lymph nodes
- Appendix
- Peyer's Patches (of GI tract)
- Tonsils
- Adenoids
- Spleen
- MALT (Mucosal-Associated Lymphoid Tissue)
  - GALT (Gut-Associated Lymphoid Tissue)
  - BALT (Bronchial/Tracheal-Associated Lymphoid Tissue)
  - NALT (Nose-Associated Lymphoid Tissue)
  - VALT (Vulvovaginal-Associated Lymphoid Tissue)

Note: Some sources place the vermiform appendix, small intestine Peyer's Patches, tonsils, and adenoids within the GALT.

Thymus

The thymus (from θυμος, thumos, Greek for warty outgrowth) is the site of T cell maturation. T cells become immunocompetent here; that is, they develop their ability to mount an effective immune response against foreign invaders without attacking the host's own tissues. The thymus lies just above the heart in the mediastinum. It is largest in childhood, and it begins to shrink significantly as a person ages. The organ itself contains two lobes, and each lobe contains numerous lobules, separated from each other by connective tissue septa known as trabeculae. Each lobule is separated into an inner medulla (with few immature thymocytes) and an outer cortex (with large numbers of immature thymocytes). In the thymus, the many different T cells (produced via somatic genetic mutation, discussed below) are exposed to MHC/Ag and MHC/self-Ag. If they do not react to MHC/Ag, they are destroyed due to their ineffectiveness (positive selection). On the other hand, if they do react to MHC/self-AG, they are destroyed in order to stop them from becoming traitorous autoimmune responses against the body's own tissue(negative selection). Only 1 out of 20 immature thymocytes will pass successfully through this vetting process and become functional T cells. Dendritic cells, macrophages, and epithelial cells are interspersed throughout both the medulla and cortex; special epithelial nurse cells surround clusters of thymocytes in the cortex.

Myasthenia gravis is an autoimmune disease where the body creates antibodies against acetylcholine receptors at post-synaptic neuromuscular junctions. In up to 25% of MG cases, there is a tumor of the thymus present (thymoma); the exact reasons for this tumor development is unknown. Additionally, removal of the thymoma will halt the disease's progress; this deserves further study as antibodies are released from plasma B cells rather than T cells, yet T helper cells may mediate the autoimmunity.

DiGeorge's Syndrome is a congenital lack of a thymus, and causes in increase in infections and a depressed immune system (especially the cell-mediated response is retarded by the lack of a thymus).

Bone Marrow

Bone marrow (medulla ossea) is the site of B cell maturation in mice and humans. B cells undergo both positive and negative selection, similar to T cell maturation in the thymus. Bone marrow is also the site of hematopoiesis, the development of the myriad blood cells from progenitor cells. The site of B cell maturation in birds is the bursa of Fabricius, after which B cells are named. The tissue of bone marrow where leukocytes, red blood cells, and platelets develop (i.e., the site of hematopoiesis) is known as myeloid tissue.

Leukemia is a cancer of the bone marrow that causes abnormal production of leukocytes (WBCs).

Acute leukemia is the biggest killer of children in the US; it results from the overgrowth of immature leukocytes in the bone marrow, retarding the production of functional WBCs, RBCs, and platelets.
Chronic leukemia is the overgrowth of relatively mature (but abnormal) leukocytes in the bone marrow.
Lymphocytic leukemia is the overgrowth of lymphoid cells (T cells, B cells, Natural Killer Cells, dendritic cells).
Myeloid leukemia is the overgrowth of myeloid cells (all blood cell types that are not lymphocytic are produced from myeloid progenitor stem cells).

Lymph Nodes

Extracellular fluid flows from capillary beds into tissue; from this tissue it enters lymphatic capillaries that are "pumped" along with the movement of skeletal muscle towards lymph nodes. The paracortical areas of the nodes contain T cells, and the central areas contain germinal centers, where B cells are contained. APCs and antigen are sent from the tissue into the lymphatics, eventually reaching the lymph nodes where they can be exposed to the T and B cell populations. This allows a faster response, as the many combinations of T and B cell specificities are able to reside in several locations throughout the body (the lymph nodes) rather than relying on random meetings of antigen and lymphocytes throughout the tissues themselves.

Spleen

The spleen acts as a site of hematopoiesis during the second and third trimesters of development, before the long bones have fully developed. In the adult, the spleen acts as a site for breakdown of dying red blood cells (lifespan 120 days). For this reason, enlargement of the spleen (splenomegaly) can occur in sickle cell anemia or in certain infections. White pulp, near the arteriolar entry points into the spleen, is where lymphocytes reside and are degraded. The central red pulp is the site of RBCs breakdown. The white pulp region has a central part, with the T cells residing in the PALS, or PeriArteriolar Lymphoid Sheath and a B cell ring (or corona) surrounding the PALS.

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